What is the most physically correct glass shader?
"Glass" is too vague; there are many kinds of glass with many different optical properties. See refractiveindex.info to get some idea of the number of parameters you can worry about.

As far as cycles is concerned, a plain glass shader is as close as you probably care to get before increasing render times too much. Though absorption may be applicable with colored glass.

While there are ways to approximate other phenomena like dispersion in cycles, if you are serious about simulating those you are likely better off using a spectral renderer like luxrender.